1. Field of the Invention
The present invention relates to a frequency-control-type piezo actuator driving circuit and a method of driving the same, and more specifically, to a frequency-control-type piezo actuator driving circuit which can measure currents consumed by a piezo actuator to automatically control a resonant frequency so that the driving efficiency of the piezo actuator can be enhanced, can control a change in the resonant frequency due to process variation and temperature change, and can be protected even when being short-circuited or open-circuited, and a method of driving the same.
2. Description of the Related Art
The current mobile phone market rapidly grows more than 20% every year. Recently, it is required to develop a mobile phone having various functions added thereto as well as a function of transmitting voice.
Accordingly, such functions as a camera, PDA, MP3 player, media player and the like are added, and mobile phones corresponding to 40% of the overall mobile phones support a camera function. Current mobile phones mainly adopt a 350 thousand pixel camera module, but more and more mobile phones have a mega pixel camera module mounted thereon. Recently, as 5 mega pixel and 7 mega pixel camera phones are consecutively launched, a function of a camera module for mobile phone is also being enhanced. Particularly, as the direct competition with a digital camera is expected, an automatic focus function and automatic optical zoom function are required. In order to meet such requirements, it is urgent to implement a lens driving actuator and driver LSI, which have low power consumption and are small in size. Recently, such a technique as a lens driving method using a piezo element attracts attention.
In the driving method using a piezo element, noise and vibration does not occur at all and power consumption can be significantly reduced to ⅓, in comparison with a conventional method using a motor.
However, the piezo element operates when resonating at a voltage of more than or equal to 10V. Therefore, if a driving signal is not accurately adjusted to the resonant frequency, the piezo element does not operate normally, or the operation efficiency significantly decreases. Accordingly, it is important to set an accurate resonant frequency.
However, since the resonant frequency band of the piezo element is very narrow and the variation of the resonant frequency is very large at the time of manufacturing, it is difficult to set an accurate resonant frequency.
FIG. 1A is a block diagram showing a piezo actuator driving circuit 100 according to the related art. FIG. 1B is a circuit diagram showing a piezo driver 120 and piezo element 130a of the driving circuit 100.
As shown in FIG. 1A, the conventional piezo actuator driving circuit 100 includes a frequency oscillator 110 which generates a resonant frequency Fres of the piezo actuator 130, the piezo driver 120 which drives the piezo actuator 130, and a piezo actuator 130 composed of four piezo elements 130a to 130d. 
The resonant frequency Fres can be generated by an oscillator provided outside the driving circuit 100, and the piezo driver 120 outputs four phase signals A1 to A4 which can drive the piezo actuator 130.
As shown in FIG. 1B, the piezo driver 120 is composed of a pulse generator 121 which generates a voltage pulse VPULSE with a constant period, a first driver stage 122 which receives the voltage pulse VPULSE of the pulse generator 121 and buffers the voltage pulse VPULSE to output, and a second driver stage 123 which converts the voltage pulse VPULSE, buffered and output by the first driver stage 122, into a current pulse to output.
The first driver stage 122, which is composed of a buffering inverter, serves to buffer the voltage pulse VPULSE generated by the pulse generator 121.
The second driver stage 123 is composed of a first current source 123a which transmits a current pulse for charging the piezo actuator 130, a first switching element 123b which receives the voltage pulse VPULSE of the pulse generator 121 and is connected to a power supply voltage VDD and the first current source 123a, a second current source 123c which transmits a current pulse for discharging the piezo actuator 130, and a second switching element 123d which receives the voltage pulse VPULSE of the pulse generator 121 and is connected to a ground voltage and the second current source 123c. 
The first switching element 123b is a PMOS transistor, and the second switching element 123d is an NMOS transistor. The first and second switching elements 123b and 123d are turned on or off in accordance with the voltage pulse VPULSE applied by the pulse generator 121 so as to generate a current pulse for driving the piezo actuator 130.
The piezo elements composing the piezo actuator 130 can be modeled by a resistance, an inductor, and a capacitor passive element. Each of the piezo elements includes an inductor stage composed of an inductor 131, a first capacitor 132, and a resistance 133, which are connected in series, and a second capacitor 134 which is connected in parallel to the inductor stage so as to resonate and is charged and discharged by the current pulse generated by the second driver stage 123 so as to maintain a constant amplitude of driving voltage pulse VACT.
In the above-described conventional piezo actuator driving circuit, however, the resonant point changes due to the temperature change or process variation of the piezo element, when the resonant frequency output from the frequency oscillator is fixed. Therefore, it is impossible to control an accurate resonant frequency.
Since an accurate resonant frequency cannot be controlled, it is likely that the driving efficiency of the piezo actuator decreases or the piezo actuator does not operate.
Furthermore, the resonant frequency output from the frequency oscillator can be controlled from the outside. In this case, since a different resonant frequency should be adjusted for each piezo actuator, the productivity decreases and the manufacturing cost significantly increases.